1. Field of the Invention
The present invention relates to an ink jet printer that applies pulse signals to an actuator of an ink jet head. The present invention further relates to a method for determining pulse width of the pulse signals applied to the actuator of the ink jet head. The ink jet printer of the present invention includes all devices for printing words, images, etc. by discharging ink towards a print medium. For example, the ink jet printer of the present invention includes copying machines, fax machines, multifunctional products, etc.
2. Description of the Related Art
An ink jet printer has an ink jet head. Usually, the ink jet head has a plurality of units, each unit having a nozzle for discharging ink toward a print medium, a pressure chamber communicating with the nozzle, and an actuator facing the pressure chamber. As one example, a piezoelectric element is used as the actuator.
A pulse signal that has at least two levels (high voltage and low voltage) is applied to the piezoelectric element. For example, a pulse signal having a high voltage, this being a base voltage, is applied. The piezoelectric element to which the pulse signal is applied changes voltage in the sequence: high voltage, low voltage, high voltage. When the piezoelectric element changes from high voltage to low voltage, the piezoelectric element deforms away from the pressure chamber. The capacity of the pressure chamber thus increases, and ink is drawn into the pressure chamber. When the piezoelectric element changes from low voltage to high voltage, the piezoelectric element deforms towards the pressure chamber. The capacity of the pressure chamber thus decreases, and pressure of the ink within the pressure chamber is increased. The pressurized ink is discharged from the nozzle. Usually, one ink droplet is discharged from the nozzle when one pulse signal is applied to the piezoelectric element.
When one ink droplet is discharged, one dot is formed on the print medium. There are ink jet printers that form one dot on the print medium by continuously discharging a plurality of ink droplets. Pulse signals are applied continuously to the piezoelectric element to continuously discharge a plurality of ink droplets. For example, two ink droplets may be discharged from the nozzle by applying two continuous pulse signals to the piezoelectric element. Usually, the ink droplet which is discharged later has a greater discharge speed than the ink droplet which is discharged first. As a result, the two ink droplets merge before reaching the print medium, and form one ink droplet. When this merged one ink droplet adheres to the print medium, one dot is formed. In this case, the size of the dot is larger than the dot formed from only one ink droplet. As another example, three ink droplets may be discharged from the nozzle by applying three continuous pulse signals to the piezoelectric element. The three ink droplets merge to form one ink droplet. When this merged one ink droplet adheres to the print medium, one dot is formed. In this case, the size of the dot is larger than the dot formed from two ink droplets.
In the present specification, a point formed on a print medium by discharging only one ink droplet from a nozzle is termed a dot. Furthermore, a point formed on a print medium by discharging a plurality of ink droplets onto the same location on the print medium from one or a plurality of nozzles is also termed a dot.
In the present specification, forming one dot from only one ink droplet is termed single discharging. Forming one dot from two ink droplets is termed double discharging, and forming one dot from three ink droplets is termed triple discharging.
The size of the dots can be changed by changing the number of ink droplets used to form one dot. There are ink jet printers which change the size of the dots according to a print mode.
Even if the same pulse signals is applied to actuators (for example, piezoelectric elements) that have been manufactured using the same manufacturing process, the ink droplets are not necessarily discharged at the same speed. For example, if the same pulse signals are applied to the piezoelectric element of one ink jet printer and to the piezoelectric elements of another ink jet printer, there may be a difference in the discharge speed of the ink droplets of the former ink jet printer and of the latter ink jet printer.
If there is a difference in the discharge speed of the ink droplets between ink jet printers, identical printing results cannot be achieved. A technique for mass-producing ink jet printers that can obtain satisfactory printing results is sought.